1. Field of the Invention
This invention relates to a semiconductor device for controlling switching power supply, which controls the output voltage of a switching power supply through switching operation.
2. Description of the Related Art
Switching power supply devices, which use switching power supply semiconductor devices to control (stabilize and similar) an output voltage by utilizing the switching operation of semiconductors (transistors or other switching elements), have come to be widely used as the power supplies of home appliances and other equipment for home use, for the purpose of improving power efficiency through reduced power consumption and similar.
In recent years in particular, attention has been paid to reduction of power consumption through operation idling (standby) in home appliances and other equipment, from the standpoint of measures to prevent global warming, and there have been strong demands for switching power supplies with lower power consumption during standby.
In order to respond to such demands, power supply systems have for example been developed in which a switching power supply for use as the main power supply which supplies power when at a rated load in the normal operating state of the equipment (normal mode), and a switching power supply for standby use which supplies power during standby when the equipment is in a standby operating state (standby mode), are provided; when the equipment is in standby, power is supplied from the switching power supply for standby, and when at the rated load power is supplied by the switching power supply for main power, so that two switching power supplies are used selectively according to the operating mode of the equipment.
Such power supply systems have the defect that two switching power supply devices (converters) are necessary, entailing increase overall costs of the circuitry, including the semiconductor devices for controlling switching power supply. Hence when it is necessary to hold down costs and in other cases, often a power supply system is adopted which comprises a single switching power supply device (converter). In such cases, partial resonance-type power supplies are often used as the switching power supply, out of considerations of power supply efficiency and noise.
However, during light loading while in standby or at other times, the semiconductor devices used in control of switching power supplies such as those described above must constantly be supplied with currents for internal circuits through the transformer, even though the current flowing in switching elements is reduced. Hence the current flowing in the switching power supply, including the current flowing in switching elements, cannot be made zero, and so even under no-load conditions, a current of a certain size flows. Hence even when there is no load, losses occur due to the switching operation in switching elements, and the lighter the load, the greater is the fraction of such losses in switching elements. Consequently the power efficiency of the switching power supply is reduced, and so there is the problem that the desire for reduced power consumption when the power supply is in standby cannot be realized.
Further, in a partial resonance-type switching power supply the oscillation frequency rises under light loads, so that switching losses are increased, and there is the problem that the power supply efficiency is lowered in standby mode.
(Prior Art 1)
As one proposal to resolve the above problem of a lowered power supply efficiency in standby mode (see for example Japanese Patent Laid-open No. 2002-315333), control technology is introduced in which the secondary-side loading state of the power supply is detected by a microcomputer, the signal thereof is received, a transition is made to standby state, and intermittent oscillation based on the commercial power frequency is executed through feedback control. In this case, in order to improve the power supply efficiency in standby mode, feedback control is executed by a microcomputer such that when the load is light and the output voltage rises to equal to or greater than a prescribed value, switching operation by the switching element is halted, and thereafter, when the output voltage falls below a prescribed value, switching operation by the switching element is resumed.
In this switching power supply, the oscillation frequency during intermittent switching operation is fixed regardless of the load state, and so this solution is far from adequate with respect to improvement of the power supply efficiency during standby.
(Prior Art 2)
To resolve the above problems, a switching power supply device such as the following is conceivable. This switching power supply device is explained below in summary fashion using FIG. 16.
FIG. 16 is a circuit diagram showing an example of one configuration of a conventional switching power supply device. As shown in FIG. 16, in this switching power supply device a DC input voltage VIN is applied to switching element 1 via the primary windings 103a of the transformer 103; through the switching operation of the switching element 1, the DC output voltage Vo, obtained by rectification and smoothing of the alternating current appearing across the secondary windings 103b of the transformer 103 by a rectifier 104 and capacitor 105, is controlled, and power is supplied to the load 109. This switching power supply device has a transformer reset detection circuit 13, which detects the reset state of the transformer 103 occurring due to the switching operation of the switching element 1 from the AC voltage occurring across the tertiary windings 103c of the transformer 103, and outputs a transformer reset detection signal indicating the reset state; an I-V converter 21, which converts a control current, obtained through the output voltage detection circuit 106 and phototransistor 110 based on the change in DC voltage Vo arising across the secondary windings 103b of the transformer 103, into a voltage corresponding to the current value; and a light-loading detection circuit 24 which, upon detecting light loading as a load state indicating the magnitude of power supplied to the load 109 based on the output voltage VEAO from the I-V converter 21, outputs a control signal to control intermittent operation of switching by the switching element 1. These portions together comprise a portion of the control circuitry which drives the control electrode (gate electrode) of the switching element 1.
When the output voltage VEAO from the I-V converter 21 becomes smaller than a light-loading detection lower-limit voltage VR1 for detection of light loading, the light-loading detection circuit 24 halts the switching operation of the switching element 1, and when the output voltage VEAO from the I-V converter 21 becomes greater than the light-loading detection upper-limit voltage VR2 for detection of light loading, the light-loading detection circuit 24 outputs a control signal to control intermittent operation such that the switching element 1 resumes switching operation. The control circuitry is configured such that the control electrode (gate electrode) of the switching element 1 is driven to control intermittent operation during light loading, based on the transformer reset detection signal from the transformer reset detection circuit 13 and the control signal from the light-loading detection circuit 24.
Operation of a switching power supply device configured as described above is briefly explained. Here, the power supply operation of a semiconductor device for controlling switching power supply, which performs intermittent operation of switching by the switching element when a light load is detected, is explained.
In FIG. 16, when the internal circuitry rises to a reference voltage the control circuitry is started, and thereafter, when the voltage at the terminal 41 rises due to the capacitor 118 connected between the terminal 41 and terminal 42 to reach the startup voltage, the power MOSFET or other switching element 1 is turned on to enter the on state; when the drain voltage thereof reaches an overcurrent detection level determined by the feedback current due to the photocoupler current flowing from the output voltage detection circuit 106 connected to the secondary windings 103b of the transformer 103 to the phototransistor 110, the switching element 1 is turned off and enters the off state. When the switching element 1 is turned off, the drain voltage undergoes ringing due to the resonance between the inductance of the transformer 103 and the drain-source capacitance of the switching element 1.
Thus, once the semiconductor device for controlling switching power supply is started, the next turn-on signal is detected by means of the tertiary windings (bias windings) 103c of the transformer 103; but within the control circuitry the bias windings voltage is clamped at a + or − level, and when the bias windings voltage falls below a preset value in the control circuitry, a turn-on signal is output. A resistance 116 and capacitance 117 are connected to the bias windings detection terminal 44, and the time constant resulting from the values of the resistance 116 and capacitance 117 is adjusted so as to obtain timing such that the switching element 1 is turned on at the bottom of the drain voltage of the switching element 1.
The above operation is repeated so as to obtain the desired output voltage Vo; but in order to improve the power supply efficiency under light loads, by executing intermittent oscillation control (intermittent switching operation) in which switching operation by the switching element 1 is halted when the feedback current equals or exceeds a certain fixed value, and switching operation by the switching element 1 is resumed when the feedback current falls below a certain fixed value, the power supply efficiency under light loads is improved and power consumption is reduced.
Quasi-resonant ringing choke converter (RCC) control is one method of control of switching operation by the switching element 1; the switching loss when the switching element is turned on can be reduced, and the noise level can be lowered, making the method suitable to market demands for low-noise, high-efficiency, high-power output. And under light loads, intermittent switching operation occurs through intermittent oscillation control, so that the increase in switching frequency under light loading which is generally a problem with RCC is suppressed, and switching losses under light loads can be reduced to some extent.
However, because a conventional switching power supply such as that described above is under RCC control, the lighter the load on the secondary-windings side of the transformer, the higher is the switching frequency, so that there are the problems that the switching loss per unit time in the switching element 1 increases, and that during light loading such as in standby mode the power supply efficiency is worsened.